INSECTS OF WESTERN NORTH AMERICA 



241 



evidence that the two resulting phylogenetic lines ever again 

 mingled geographically. 



In addition to these mid-Cenozoic events which can be linked 

 ecologically with temperate forest dispersals, we have evidence of 

 other intercontinental dispersals of cool-adapted insects that are 

 difficult to date. An example (Ross, 1956) is the deduced dispersal 

 of the caddisflies belonging to the Rhyacophila sibirica group (Fig. 



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Fig. 7. Phylogenetic dispersal chart of the Rhyacophila sibirica group, 

 comprising caddisflies restricted to rapid, cold streams. (From Ross, 1956.) 



7). Its main evolutionary lines seem to have oscillated between 

 northwestern America and northeastern Asia, with many surviving 

 species in both areas. The distribution of these chiefly northern 

 montane species indicates the possibility of some interchanges in the 

 northern elements of our western cool-adapted biota at moderately 

 frequent intervals during the Cenozoic. For other groups of montane 

 caddisflies of indicated Asiatic origin, a single line seems to have 

 spread to and to have become established in western North America. 

 Examples are Himalopsyche phryganea (Fig. 8), Glossosoma penitum, 

 and the entire Rhyacophila acropedes complex. Beacuse each line 



